One of the main themes is often the power of friendship. Even when captured, Harry's friends would be fighting for him. Also, the theme of good versus evil is prominent. Harry represents good, and Voldemort is the ultimate evil. The fanfic might show how Harry resists the dark influence of Voldemort. Additionally, themes like hope and perseverance are likely to be there. Harry doesn't give up easily, and that gives hope to the wizarding world that he can still overcome Voldemort, even when in captivity.
Well, the theme of identity is quite common. Harry is constantly trying to define himself against Voldemort, and Voldemort's own twisted sense of self is also explored. Another theme is the idea of a second chance. Some fanfics give Voldemort a second chance at life or at changing his ways. And the theme of the past's influence is there too. How the past actions of both Harry and Voldemort's families and predecessors impact their present - day relationship.
Power is a common theme. Since both Harry and Voldemort are associated with great power, many fanfics explore how they use or abuse it. Another one is redemption. Some fanfics like to imagine if Voldemort could be redeemed.
In such a fanfic, Harry might be tortured for information about the Order of the Phoenix. Voldemort could try to turn Harry to the dark side, using his powerful magic to break Harry's will.
One common theme is the power struggle. Since both Harry and Voldemort are powerful in their own ways, fanfics often explore how their relationship affects the balance of power in the wizarding world.
Power is a common theme. Voldemort desires ultimate power, Severus is often caught in the power struggle between different sides, Lucius wants to regain his family's power, and Harry Potter is the one who stands against Voldemort's tyrannical power.
Power is a very common theme. Voldemort is all about power, so many fanfics explore how power corrupts or how characters gain and lose power in relation to him.
Power is a common theme. Fanfictions often explore how Voldemort amasses and uses his power, and how others react to it. For example, some stories might show characters trying to resist his power grab.
Well, a major theme is the rediscovery of magic. In the original series, a lot of magic was used in the fight against Voldemort. In these fanfictions, magic can be used for more positive, creative things. Also, friendship is often explored in a different light. Without the dark shadow of Voldemort, friendships can form and develop based on different interests and personalities rather than just being part of a resistance against the Dark Lord.
In such a fanfiction, Harry might be tortured for information about the Order of the Phoenix. Voldemort could try to turn Harry to the dark side, using threats against his friends.
Another theme might be manipulation. Voldemort could be hugging Harry as part of a devious plan. He might be trying to lull Harry into a false sense of security or gain some sort of advantage over him, like getting closer to the source of his power. It's a way to add a new layer of intrigue to the relationship between the two characters.